Incandescent light sources are brighter and more efficient the closer their temperature is to 6500 K (about 6200 degrees Celsius). At this temperature, the human eye can see in the form of visible light about forty percent of the energy expended by the source. This is the maximum amount of energy visible from an incandescent light source.
Solid, man-made incandescent light sources can not operate at this brightness and efficiency, however. This is because solid materials cannot be heated to 6500 K (the surface of the sun is 6500 K, but it is made up of plasma, not solid materials).
Various notable scientists and engineers have struggled to determine what solid material makes the best light source. In 1800, Humphry Davy invented the first electric light but it had a very short lifespan. Much later, physicist Sir Joseph Swan (c. 1860) developed a longer-lasting electric light with a carbon paper filament. Unfortunately, Sir Swan's design also had a short lifespan. Thomas Edison later developed (c. 1879) a filament made of carbon black (elemental, simple carbon) coated over a piece of string. His early design lasted 40 hours in an oxygen-free bulb. Lewis Latimer, a member of Edison's research team, later patented a method for manufacturing carbon filaments in 1881. Carbon-coated filaments, however, were ultimately found to be inadequate because of their low reliability and low operating temperature.
William Coolidge (c. 1910) later used tungsten filaments. Tungsten filaments were found to have a longer lifespan than Edison's carbon-coated filaments. Tungsten has the second-highest melting point of any material, about 3700 K, allowing it to be heated to a high temperature. Tungsten, however, is not capable of being heated to near 3700 K without rapidly failing. It, like most materials, vaporizes at a faster rate the closer its temperature is to its melting point. Because of this, there is a trade off between a material's efficiency, which is enabled by high-temperature operation, and its lifespan. Many currently used tungsten-filament devices operate at a temperature of about 2800 K to give them an acceptable lifespan and efficiency. At this temperature they are about ten percent efficient—about ten percent of the energy expended is visible to the human eye.
The lifespan of a filament can also depend on the filament's structure. If a filament has a polycrystalline structure, for instance, there may be localized heating at the crystal boundaries. Localized heating increases the temperature around the crystal boundaries, causing these areas to fail more quickly, such as by melting or vaporizing faster due to their higher temperature. Tungsten filaments often have a polycrystalline structure, which can limit their lifespan.
There is, therefore, a need for radiation and/or incandescent light-emitting devices having a higher efficiency and/or longer lifespan.
The same numbers are used throughout the disclosure and figures to reference like components and features.